Little is known about the mechanisms controlling bronchial blood flow or the relationships between bronchial blood flow and such tracheobronchial functions as mucus secretion, ciliary clearance and heat and water exchange. Pilot experiments in this laboratory have demonstrated the feasibility of measuring bronchial blood flow with an electromagnetic flowmeter in the sheep. Thus, a systematic study of the bronchial circulation is proposed. In the first year of this study, the blood flow through the large artery (1.0 mm diameter) that runs from the ventral surface of the aorta to the trachea at the level of the carina (Carinal artery) will be measured and compared with the simultaneously measured flow in the main pulmonary artery in the chloralose anesthetized sheep. To determine the portion of the total bronchial bed supplied by the carinal artery, Sr-85 labelled microspheres will be injected directly into this artery while Ce-141 labelled microspheres will be simultaneously injected into the aorta. These data will be correlated with data obtained from multicolored casts made by injecting Microfil into the carinal and pulmonary arteries and aorta at physiological pressure after heparinization and exsanguination. At the conclusion of these experiments and continuing into the second year, these data will be used in the conduct and interpretation of experiments exploring the hemodynamic relationships between the pulmonary and bronchial circulations. Pressures in the arterial and venous segments of the pulmonary and systemic circulations will be varied systematically and the carinal and pulmonary blood flows will be measured. In addition, the ability of the bronchial circulation to autoregulate blood flow over a range of perfusion pressures and the local responses to hypoxia and cold will be examined. In the third year, the reflex control of the bronchial circulation will be studied. Receptors to be stimulated include the arterial baro- and chemoreceptors and the responses to hypothermia. Future experiments will be directed towards developing a chronically instrumented preparation to study the correlation between bronchial blood flow and tracheobronchial function.